Scanning of a scale graduation in known position measuring systems, for example on an optical basis, is customarily performed with the aid of a scanning unit. Initially the scanning unit provides analog scanning signals, for example in sine or cosine form, which are further processed to provide position evaluation. There are several options for the further processing of these signals. For example, position measuring systems with analog as well as with digital signal output exist, where an electronic evaluation system which processes the analog signals is partially integrated into the measuring system. It is also possible to digitize the analog scanning signals in the scanning unit. TTL signals, for example, then result at the output of such a position measuring system which can be supplied to the numerical control of a machine tool.
For varying purposes it is now of interest to the respective user to be able to check the originally generated analog scanning signals, usually sine or cosine shaped, in order to perform or control the correct mounting of the scanning unit. Furthermore, such an option during the actual measuring process also makes sense in order to detect possible malfunctions in the system early on. In this connection the amplitude or the respective signal offset of the generated analog signal, for example, are of interest in order to be able to evaluate the signal quality. Furthermore, other information specifically of the measuring system can be of interest for the user, for example, the position of reference marks or information regarding the status of the scanning light source, etc.
An error report device associated with the scanning unit is known from German Patent Publication No. DE 38 29 815, which monitors the generated analog scanning signals and transmits error recognition signals to a downstream connected evaluation unit through a separate error signal line. Activation of the error reporting device is accomplished by sending a defined activation signal over the error signal line from the evaluation unit to the scanning unit. With this solution at least one additional transmission line is necessary both for the transmission of the activation signal and the actual error signal.
Other arrangements which have one or several such transmission lines for transmitting error-related information or their activation in connection with analog scanning signals are also described in U.S. Pat. No. 4,423,958 and German Patent Publication No. DE 34 45 617.
It is also known to perform such checking of the analog scanning signals generated by the scanning unit, including the error indication, by use of colored LEDs directly at the scanning unit as described in U.S. Pat. No. 5,241,173. However, the information regarding the quality of the analog scanning signals is not available for further processing in the evaluation unit. Thus there is only a limited usefulness of this information, in particular with regard to the correct mounting of the scanning unit.